1. Field of Technology
The present specification relates to transmitters generally, and to delta sigma modulator based transmitters more specifically.
2. Related Art
Due to the excessive demand for higher data rates and more communication services, spectrum efficient modulation and multiplexing techniques, such as code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM), are widely used to cope with the ever-increasing cellular traffic within wireless communication networks. These advanced techniques, however, may give rise to communication signals with high peak-to-average-power-ratios (PAPRs). To avoid distortions, the power amplifier (PA) of the wireless transmitter may be required to operate at large back-off from the saturation region, which results in low average efficiency performance.
The delta sigma modulator (DSM) based transmitter is an architecture capable of providing average efficiency enhancement of radio frequency (RF) transmitters. Conventionally, a DSM based transmitter includes a two-level DSM unit that quantizes a time-varying input signal, a signal up-converter that shifts the frequency of the quantized signal to the desired radio frequency (RF) carrier frequency, and a switching mode PA (SMPA) that boosts the power of the RF signal before transmission. Given that the quantized signal has two levels, the envelope of the RF signal feeding the SMPA is constant. As such, no output power back-off may be required; and, the SMPA can be driven at saturation at all times, which may maximize its efficiency. Furthermore, there may be no need for any predistortion techniques, which may reduce the complexity of the digital signal processing part of the transmitter and therefore reduces its energy consumption.
The overall efficiency of DSM based transmitters is determined by the product of the coding efficiency (CE) of the DSM times the peak efficiency of the SMPA. The CE is the ratio between the in-band signal power to the overall quantized signal power. In the case of two-level DSM, the CE is critically low. Thus when the SMPA saturates there is a very low desired output power. As a result, the overall efficiency of two-level DSM based transmitters is very limited.
To surmount this problem, multi-level (three- and higher-level) DSM based transmitters have been proposed. The use of multi-level DSM allows increasing the CE of the quantized signal, which in principle, should significantly improve the overall efficiency of multi-level DSM based transmitters compared to their two-level DSM based counterparts. However, since the quantized multi-level signal is no longer constant-envelope, the SMPA is operated at back-off region most of the time. As such, the efficiency enhancement is restricted by the average efficiency of the SMPA. Furthermore, the use of a linearization technique is required in order to recover the linearity of multi-level DSM based transmitters.